Waterflooding



3,042,611 Patented July 3, 1962v 3,042,611 WATERFLOODING John T. Patton,Tulsa, Okla, assignor to Jersey Production Research Company, acorporation of Delaware No Drawing. Filed May 1, 1959, Ser. No. 810,2514 Claims. (Cl. 252-855) The present invention is broadly concerned withthe recovery of petroleum from underground reservoirs. Moreparticularly, it concerns a method of waterflooding with a flood watercontaining: (1) calcium carbonate in a quantity suflicient to saturatethe water and (2) a polysaccharide derived from sucrose by enzymaticreaction in a quantity sufiicient to increase the viscosity of thewater. The invention is especially concerned with awaterflooding'process in which the flood water is a viscous aqueoussolution of dextran, saturated with calcium carbonate, and containing abactericide--especially formaldehyde. The quantity of bactericide shouldbe sufficient to reduce the tendency of the aqueous solution to degradeviscosity-wise.

It is a well-known practice in the petroleum industry to waterfloodsubterranean petroleum reservoirs in order to increase the recovery ofpetroleum therefrom. Thus, it is a common practice to inject Water intosuch reservoirs through one or more injection wells so as to displaceand recover oil from the reservoirs through one or more production wellsspaced from the injection wells.

While conventional waterflooding is elfective in obtaining additionaloil from underground reservoirs, it has a number of shortcomings whichdetract from its value. Among these is the tendency of flood Water tofinger through by-pass substantial portions of a reservoir. In otherWords, a waterflood or water drive usually has a less than perfect sweepefficiency in that the water does not contact all portions of thereservoir. Furthermore, much of the recoverable oil is not normallydisplaced even from those portions of a reservoir that are actuallycontacted by the flood water.

The fingering tendency of a Waterflood is usually explained by the factthat oil reservoirs possess regions or strata that have difierentpermeabilities. Some of these regions and strata have a greater relativepermeability to water than do other portions of the reservoir with theresult that flood water flows more readily through these regions andstrata. This causes an'inefficient displacement of the oil by the Water.

Petroleum crude oils vary greatly in viscositysome being as low as oneor two centipoises (cp.) and some ranging up to 1000 centipoises or evenmore. This information is of interest, since waterflooding is generallyless satisfactory with viscous crude oils than with relativelynon-viscous oils. In other words, the fingering and by-passingtendencies of flood water are inversely related to the ratio of theviscosity of the flood water within a reservoir to the viscosity of thecrude oil. In fact, this mathematical relationship has proven helpful inexplaining the behavior of fluids flowing through porous media such aspetroleum reservoirs. The relationship shows that oil is displaced moreefiiciently by water when the waterto-oil viscosity ratio is increased.

The water-to-oil viscosity ratio existing within a given reservoirafiiords a measure of the volume of flood water required inwaterflooding the reservoir to reduce its oil content to a particularvalue. Thus, a 'given volume of flood water operating at'a water-to-oilviscosity ratio of one will displace a markedly greater volume of oilfrom a reservoir than will an equal volume of water operating at awater-to-oil viscosity ratio substantially less than one.

The significance or importance of the water-to-oil viscosity ratio inwaterflooding operations has led a number of persons to use or suggestthe use of Water-soluble polymers, chemicals, and other thickenermaterials to increase the viscosity of flood water. These techniques,however, have met with variedand usually limitedsuccess. For the mostpart, they have the disadvantage of requiring large amounts of expensivematerials. Furthermore, many of the solutions tend to plug earthformations; and they are often degraded to an undesirable extent bytemperature, light, and by the presence of ma terials naturallyoccurrring within reservoirs. For example, reservoir sands and the ionspresent in most reservoir connate waters frequently have a very adverseeflfect upon the stability of such solutions. Also, the thickenermaterials are often adsorbed by reservoir rock surfaces and are therebyremoved from solution.

It is, accordingly, a general object of this invention to provide meansfor increasing the viscosity of flood water used in waterfloodingoperations by the addition thereto, or the formation therein, of athickening agent which overcomes the difiiculties associated with thethickening agents that have been previously used or suggested for use insuch operations. It is a further object of the invention to provide awaterflooding process using water of increased viscosity wherein theviscosity of the Water has increased stability within petroleumreservoirs. It is still a further object of the invention to provide amethod of increasing the viscosity of flood waterby incorporatingmaterials within the water which render the water viscous; which do notdegrade within reservoirs; and which are economical, readily available,and do not tend to plug reservoirs.

These and related objects of the invention, which will be expresslydiscussed or readily apparent from the following description, may beattained by using flood water containing a polysaccharide derived fromsucrose by enzymatic reaction and in an amount suflicient to increasethe viscosity of the water. The flood Water also must contain suflicientcalcium carbonate to saturate the flood water; and it should furtherpreferably contain'a suflicient amount of a preservative of a characterto prevent the growth of microorganisms which would otherwise tend todegrade the polysaccharide. The polysaccharide, the calcium carbonate,and the preservative are incorporated within the flood water before itis injected within a reservoir. Conventional techniques may be usedforactually injecting the water through one or more input Wells into areservoir and for recovering petroleum thus displaced from the reservoirthrough one or more output wells spaced from the input wells.

In incorporating a polysaccharide thickener of the invention to aflood'water, the desired concentration of thickener may be'obtainedsimply by dissolving a commercially available material of this typewithin the water; Alternatively, the thickener may be grown ormanufactured directly at the reservoir site through the use of suitablenutrient aqueous sucrose solutions together with suitablemicroorganisms. In either case, sufficient calcium carbonate is added tothe water to saturate it. A preservative such as formaldehyde is alsopreferably added to the water. v

In practicing this invention, the use of many conventional'polysaccharide thickeners and preservatives will occur to personsskilled in the art. However, the best mode contemplated for carrying outthe invention comprises incorporating sufficient dextran within a floodwater to obtain a preselected viscosity for the water, and alsoincorporating a sufficient amount of calcium carbonate 3 within thewater to reduce degradation of the dextran. Formaldehyde is alsopreferably added to further reduce degradation of the dextran.

Dextran has been found to be thermally stable over a wide range ofreservoir temperatures, and it has a substantial thickening effect onwater. It is neither adsorbed nor degraded by contact with rocksurfaces; and the ions present Within most reservoirs have little effectupon it. It is also substantially non-plugging. Formaldehyde, on theother hand, has been found to have not only a pronounced bactericidialeffect within dextran solutions, but also a pronounced stabilizingchemical effect. In other words, formaldehyde not only protects adextran solution against the effects of microorganisms such as bacteriaand molds, it also greatly improves the chemical stability of thedextran at reservoir temperatures and in the presence of rock surfaces,ions, etc.

The action of calcium carbonate on aqueous solutions of dextran andother sucrose-derived polysaccharides is not entirely known orunderstood. However, its presence is known to have a stabilizing effectupon such solutions especially in the presence of a preservative, andparticularly formaldehyde.

As is well knownin the art, dextran is conventionally derived fromaqueous sucrose solutions by the action of microorganisms such asLeuconostoc mesenteroides. Once formed, the dextran may be precipitatedfrom the resulting solution as by the addition of alcohol, and theprecipitated dextran then filtered or otherwise separated from thesolution.

In the practice of the invention, it is contemplated that dextranmanufactured and separated in the manner described above may be added toa flood water in the amount required to obtain the desired viscosity.The best mode of practicing the invention, however, is contemplated tocomprise generating the dextran by enezymatic reaction directly within aportion of the flood water to form a thickened solution directly at thereservoir site. It is further contemplated that dextran may be grown ina portion of the flood Water and that this portion may then be blendedwith the main body of flood water preparatory to its injection within areservoir.

The amount of dextran to be added to a flood water in any givenreservoir may vary considerably. In general, of course, the amount ofdextran should be such as to increase the flood waters ability todisplace oil from the reservoir. Preferably, the quantity of dextranshould be sufficient to endow the flood water with a mobility within thereservoir which is equal to or less than the mobility of the reservoiroil. As a practical matter, the method of this invention has applicationparticularly to those reservoirs in which the oil has a viscosity of atleast about five cp. When practicing the invention in such reservoirs,then, the dextran-thickened water would normally have a viscosity of atleast about five cp.--and preferably at least enough to provide theflood water with a mobility about equal to that of the reservoir oil.

In incorporating dextran within a body of flood water, it may beincorporated within the entire body of water. The best mode contemplatedfor incorporating the dextran, however, is to incorporate it merelywithin the forward or leading portion of the flood. When limiting thepresence of dextran to the leading portion of the flood, the volume ofwater thicknened by the dextran should preferably be sufiicient toprevent the trailing, relatively non-viscous water from breaking throughthe viscous water and contacting reservoir oil ahead of the viscouswater. In this connection, it is contemplated that a diminishingconcentration of dextran from the leading edge or front of a flood to atrailing portion of the flood is the best procedure for avoiding apremature breakthrough of water when the entire body of water is notthickened.

The minimum quantity of viscous water to be used in any given reservoirwill depend upon such well-recognized factors as the flooding pattern,the distance between wells,

the viscosity of the crude oil, etc. In general, however, it iscontemplated that the volume of viscous water should be at least 10percent of the reservoir pore volume under flood, and preferably atleast 20 percent of the pore volume.

It is well to note at this point that it is conventional in thepetroleum industry to carry out waterflooding operations using certainregular flooding patterns. Thus, the volume of a reservoir under floodat any one time may be considered to be the volume of the reservoirunderlying the lateral area defined by the patterns under flood at suchtime. Expressed otherwise, it is general practice in the petroleumindustryespecially in large reservoirs or fieldsto flood using one ormore flooding patterns. It may therefore be convenient, in applying abank-type version of the present invention, to consider the individualpatterns within a reservoir in determining the quantities of viscouswater to employ. To illustrate, in line-drive floods, it may be assumedthat the pore volume of a reservoir under flood between each row ofinjection wells and an adjacent row of producing wells is the total porevolume of the reservoir underlying the lateral area between the tworows. The quantity of dextran containing water injected within a row ofinjection wells toward an adjacent row of producing wells should then beat least 10 percent of the total reservoir volume between the two rows,and preferably at least 20 percent.

The quantity of formaldehyde or other preservative employed to stabilizethe viscosity of a dextran solution in any given reservoir may bedetermined experimentally outside the reservoir. For example, knowingthe temperature of the reservoir and having samples of the reservoirsand, it is possible to carry out laboratoryscale tests whicheffectively duplicate reservoir effects and conditions. In general, ithas been found that quantities of formaldehyde in the range of about 0.1to 5 percent by volume of a dextran solution are very effective for thepurposes of the invention.

The flood water may be saturated with calcium carbonate in a variety ofways, as for example by dispersing finely ground limestone throughoutthe water and then removing any undissolved portion by a simple settlingprocess. However, it is contemplated that the best mode of obtaining asaturated solution is to filter the water through a bed of limestoneparticles. It is conventional practice in waterflooding operations tofilter flood water before injecting it within a reservoir. Thisoperation, then, can be readily modified to include the step ofsaturating the water with calcium carbonate.

To recapitulate briefly, then, the best mode contemplated for practicingthe invention comprises first forming a polysaccharide such as dextranby enzymatic reaction within a portion of the water to be used in awaterflooding operation. After the polysaccharide has been formed,degradation of the polysaccharide is guarded against by the addition ofa preservative-preferably formaldehyde. The preservative preferablyshould not be of a type such as potassium permanganate or potassiumdichromate which exhibits an oxidizing elfect. The resulting mixture isdiluted with additional unthickened flood water to obtain a blend havinga mobility within the reservoir under flood about equal to that of theoil within the reservoir. Immediately prior to its injection within thereservoir, the flood water is filtered and saturated with calciumcarbonate.

To further illustrate the invention, attention is directed to thefollowing examples.

EXAMPLE 1 In a first example, IOO-milliliter (ml) portions of variousaqueous dextran solutions were held at F., F., or F. for periods up tofive weeks. The viscosities of the solutions at 80 F. were periodicallydetermined and noted. The aqueous medium in each instance was asimulated lake water containing 418 parts and the results obtained fromthe tests, are presented below.

Table l Viscosity (ep.) at 80 F. Aging Sample E -fi Start 1 Week 2 Weeks3 Weeks 5 Weeks 2% dextran 150 26. 6 4.0 8. 2.4 2% dextran 1% formalin150 28. 2 14. 8 12. 8 12. 4 10. 0 2% dextran 1% torma 10 p.p.m.051303...- 150 27.0 16. 0 15. 6 17. 0 16.0 2% dextran 0.032% Zepliiran10 p.p.m. CaCO; 150 24. 4 7.0 3. 0 2% dextran 0.005% HgClz 10 p.p.m.CaCO 3. 150 27.6 6. 0 2. 6 2% dextran 10 p.p.m. CaCOa.... 150 33.6 8.82% dextran 2% formalin 130 27. 8 15. 9 16. 1 17. 1 18.0 2% dextran 1%formalin 10 p.p.m. 03003...- 130 22.0 17.5 17.6 17.1 2% dextran 80 24. 020. 4 17. 6 20. 2 19. 2% dextran 1% formalin 80 24.0 22. 0 21. 4 20. 222. 8

l Benzalkonium chloride.

The above results show that dextran solutions tend to degrade and loseviscosity with increasing time and temperature. The results further showthat CaCO and various preservatives tend to reduce this degradation andthat .the effect of the preservatives is enhanced by the presence ofcalcium carbonate.

EXAMPLE 2 In this example, 100-ml. portions of various aqueous dextransolutions were aged at 70 to 75 FJfor periods up to forty-three days.The aqueous medium was a brine containing 58,000 p.p.m. sodium; 9,700p.p.m. calcium; and 107,300 p.p.m. chloride. In some instances, 100grams of a loose reservoir sand were added to the solutions. The resultsof these tests are shown in the following table.

Table II Viscosity After Aging at 7075 F.Centistokcs Test SolutionInitial 5 days 12 days 19 days 29 days 43 days 1.5% dextran 13.3 13. 613. 2 13.2 1 13.4 1 12.6 1.5% dextran +100 ,2. sand 13. 3 11. 8 12.9 210. 8 1 9. 9 3 10. 6 1.5% dextran +100 g. sand +0.5 g. mixture 2/1formalinacetaldehyde** *13. 3 11.8 11.2 13. 3 12. 3 13. 5 1.5% dextran+100 g. sand +1.0 g. mixture 2/1 formalinacetaldehyde *13. 3 12. 4 12. 63 13. 8 3 12. 8 15.1 1.5% dextran +100 g. sand +2 ml. Clorox 5. 9 5. 16.0 5. 1 5. 0 6. 1

1 day 3 days 7 days 14 days 17 days 1.5 dextran +1 0 I'Zrmalin i 13. 613.0 12.9 12. 4 12. 8 12.8 1 5% dextran formalin +100 g. sand 13. 6 12.9 13. 1 12. 6 13. 6 13. 4

*Estimated. "Acetaldehyde in 50% solution. 1 White precipitate. I Moldformation.

3 Cloudy.

The above results show several interesting features. First, they showthat aqueous dextran solutions-in the absence of preservatives and inthe presence of sand are subject to degradation as evidenced by theformation of mold. They als show that this type of degradation issuccessfully avoided by the presence of a preservative such asformaldehyde.

EXAMPLE 3 In a third example, a 2% aqueous dextran solution was passedthrough a tube packed with an unconsolidated reservoir sand. The aqueousmedium was the same type as that used for the data in Table I. The tubewas one inch in diameter and about two feet long. The test was carriedout at room temperature, and sufficient solution was passed through thetube to produce 2.2 pore volume of efliuent. The efiluent wasperiodically examined for its viscosity and its dextran content. Theoriginal solution had a viscosity of about 16.3 cp.

In this experiment it was found that the efliuent had a dextranconcentration substantially identical with that of the originalsolution. It was further found that the efl1uentexcept for the first 0.2pore volume-had a viscosity substantially identical with that of theinitial solution. This was clear evidence that the viscosity andconcentration of aqueous dextran solutions at room temperatures aresubstantially unafiected by contact with reservoir sand.

While the foregoing description has been devoted largely to the use offlood waters containing dextran, calcium carbonate, and formaldehyde, itwill be recognized that modifications of this concept may be employedwithout departing from the spirit or scope of the invention. Forexample, it is contemplated that preservatives other than formaldehyde,such as acetaldehyde and other conventional bactericides and germicides,may be employed. Quaternary ammonium compounds, for example, areeffective, but not nearly so effective as formaldehyde. It is alsocontemplated that the preservative material need not alway be agermicide or bacten'cide, although it is preferably so. Again, it shouldbe noted that the expression dextran solution is intended to includedextran dispersions, since true solutions of dextran may not alwaysexist.

What is claimed is: l

1. In a method of recovering oil from a subterranean oil reservoir, thestep of flooding the reservoir with water containing an amount ofdextran suflicient to increase the viscosity of the water, said waterbeing further saturated with calcium carbonate and containing a quantityof preservative sufficient to reduce the tendency of the dextran todegrade.

2. In a method of recovering oil from a subterranean oil reservoir, thestep which comprises flooding the reservoir with water containing anamount of dextran sufiicicut to increase the viscosity of the water,said water further being substantially saturated with calcium carbonate4. A method as defined in claim 3 in which the water contains sufiicientdextran to provide the water with a mobility within the reservoir whichis no greater than that of the reservoir oil.

(References on following page) 3,042,611 7 I 8 a References Cited in thefile of this patent 2,738,325 Rydell Mar. 13, 195 6,

UNITED STAT FATE 2,771,138 13665011 NOV- 20, 1956 2 341 500 D 11 ES F b8 1944 2,868,725 Owen Jan. 13, 1959 4 et 'ng e 2,360,327 Bailey et al.Oct. 17, 1944 5 OTHER REFERENCES 2,364,434- Foster Dec. 5, 1944 Rogers:Composition and Properties of Oil Well Drill- 2,602,082 Owen July 1,1952 ing Fluids, Revised Ed., pub. 1953 by Gulf Pub. Co. of 2,731,414Binder et a1 Jan. 17, 1956 Houston, Texas, pages 407, 420 and 421.

1. IN A METHOD OF RECOVERING OIL FROM A SUBTERRANEAN OIL RESERVOIR, THESTEP OF FLOODING THE RESERIOR WITH WATER CONTAINING AN AMOUNT OF DEXTRANSUFFICIENT TO INCREASE THE VISCOSITY OF THE WATER, SAID WATER BEINGFURTHER SATURATED WITH CALCIUM CARBONATE AND CONTAINING A QUANTITY OFPRESERVATIVE SUFFICIENT TO REDUCE THE TENDENCY OF THE DEXTRAN TODEGRADE.